1. Field of the Invention
The present invention relates to the field of inductances, and more specifically to the forming of an inductance having its value modified according to the frequency of the signal which is applied thereto.
2. Discussion of the Related Art
An example of application of the present invention relates to radiofrequency transceiver chains, for example, of the type used in bi-band mobile phones, that is, capable of operating on two frequency bands (for example, GSM and DCS).
FIG. 1 very schematically shows in the form of blocks a conventional example of a conventional bi-band radiofrequency transceiver system to which the present invention applies. The architecture of such a system can be described as a receive chain 1 and a transmit chain 2, both connected to an antenna 3.
On receive side 1, two parallel receive paths each providing a signal RX1 or RX2 according to the frequency band at which the signal is received, directed to an interpretation system not shown, generally a digital system, are available. Each path comprises, between a terminal 11 or 12 of connection to antenna 3 via a band selection switch 13:
a band-pass filter 111 (BP 1) or 121 (BP 2) centered on a frequency of the corresponding band;
a low-noise amplifier (LNA) 112, 122; and
a mixer 113, 123 of the signal output by the preceding amplifier with a signal provided by a local oscillator OL1, OL2 at the band frequency of the concerned path. The respective outputs of mixers 113 and 123 provide received signals RX1 and RX2.
For a proper operation of receive chain 1, it is necessary to provide, between each component of the chain, an impedance matching element (ZA), generally at 50 ohms. Thus, elements 41 are respectively provided between filter 111 and amplifier 112, between amplifier 112 and mixer 113, and at the output of mixer 113, upstream of the interpretation system of received signals RX1. Similarly, for the second path, impedance matching elements 42 are respectively provided between filter 121 and amplifier 122, between amplifier 122 and mixer 123, and at the output of mixer 123. Elements 41 and 42 differ by the central frequency of the concerned band.
The transmit chain side has substantially the same architecture, that is, each path includes, between the system for providing a signal TX1 or TX2 to be transmitted and a terminal 21 or 22 intended to be connected, by a switch 23, to antenna 3:
a mixer 213, 223 of the signals to be transmitted with a signal provided by a local oscillator OL1, OL2 at the central frequency of the passband of the considered path;
a power amplifier (PA) 212, 222; and
a band-pass filter 211 (BP1), 221 (BP2), centered on the central frequency of the passband of the considered path.
As for the receive chain, it is necessary to provide impedance matching elements 41 between amplifier 212, and filter 211, between mixer 213 and amplifier 212 and at the input of mixer 213, and elements 42 between amplifier 222 and filter 221, mixer 223 and amplifier 222, and at the input of mixer 223.
FIG. 2 shows a conventional example of an impedance matching element 4. Such an element is generally formed of two input and output terminals 43 and 44 of element 4, of a capacitor C4, and of an inductance L4. Inductance L4 grounds an electrode of the capacitor (connected to terminal 44). The capacitor and the inductance of elements 4 are sized so that at the work frequency (central frequency of the passband desired for the system), the impedance matching system exhibits on one side the conjugated complex impedance of the circuit to be matched (for example, a transistor), when loaded on the other side with the desired impedance (for example, 50 ohms). Generally, the impedance of the circuit to be matched varies according to frequency. Further, for the same impedance to be matched, the value of the elements varies according to the working frequency.
It can thus be seen that, to form a bi-band transmit or receive chain, the impedance matching elements must be sized differently according to the considered path.
This need for separate impedance matching elements adversely affects the miniaturization of electronic circuits (for example, radiofrequency transceiver chains in the considered example).
In this application, the fact that the impedance matching elements are dedicated to each frequency band requires using separate amplifiers and mixers for each frequency band. This problem is particularly present in the case where the transmit or receive amplifiers are integrated, with no impedance matching element.
More generally, in any bi-band or multi-band application for which an inductive element is sized according to the frequency of the concerned band and cannot adapt to the frequency of another band, a bulk problem arises for the electronic circuit due to the necessary size of the inductive elements. This problem is particularly present for the forming of inductive elements in an integrated circuit formed of planar windings on a surface of an integrated circuit or of any substrate.